Cultures of a single cell clone of mouse mammary tumor cells (Mm5mt/c1) that initially synthesized only MMTV (type B retrovirus) produced increasing amounts of type C retrovirus during passage in culture with some reduction in MMTV production. This was accompanied by a marked increase in the synthesis of type C viral RNA but not by decrease in MMTV RNA. Mouse interferon inhibited the production of both viruses equally without a significant effect on intracellular concentrations of two viral RNAs. The restriction analysis of polyoma virus genome in nude mouse mammary tumors showed that viral genomes were organized as tandem repeats integrated in the cell genome. Some of the tumors also contained free copies of viral DNA and some viral DNA molecules had deletion covering 0.12-0.18 map units. A comparison of the sequence complexity and diversity of mRNA in cultured human breast and prostate carcinoma cells showed that about 80% of the sequences were held common. The abundant mRNA species of one cell were less frequently represented in the other cell but only by a factor of two to four.